The invention relates to a transparent, preferably three-layer, biaxially oriented polyester film having a smooth surface A which has substantially no external particles, and a rough surface C which contains particles in a defined concentration and in a defined size. To attain a very high surface smoothness of the surface A, the film is highly oriented. The film is suitable in particular for barrier coatings which are applied by the CVD (chemical vapor deposition) or by the PECVD (plasma enhanced chemical vapor deposition) method. The invention further relates to a process for producing the film and to its use.
Films having a smooth surface A are disclosed by the prior art.
For instance, films in accordance with examples 4 and 6 of EP-A-0 903 222 contain no external pigments in the surface layer A. The film accordingly has a smooth surface A. According to experiments carried out there, the film has good barrier values, in particular after metallization or coating with ceramic materials. For barrier coating by the CVD or the PECVD method, the smoothness of the surface A to be coated is in need of improvement. Also in need of improvement is the processing performance of the film, in particular on high-speed machines. The film according to the aforementioned examples 4 and 6 is not adequately pigmented on the surface layer C on the opposite side to the surface layer A. In the case of coating in high-vacuum units, for example in units which work by the CVD or by the PECVD method, the unit is evacuated before the coating. This also allows air present in the film roll to escape, resulting in the individual film plies lying in close contact. Inadequate pigmentation may result in poor running performance of the film in the machine (pulling on the bias, blocking). The winding quality of the coated film is unsuitable for the further processing of the film (lamination, printing).
DE-A-16 94 404 describes a film composed of layers of an oriented crystallizable thermoplastic film, in which at least one of the outer layers contains an additive. The additives are customary inert inorganic or organic particles which, in the case of inert particles such as SiO2 are added to the outer layers in a concentration of from 1 to 25% by weight. The particle size is from 2 to 20 μm. These laminates can be used, for example, for decorative purposes metalized with aluminum or for magnetic tapes. There is no information whatsoever in the document as to how the topography of such a film can be adjusted for the improvement of the oxygen barrier.
DE-A-22 30 970 describes a magnetic recording medium which consists of a biaxially oriented polyester film and a thin magnetic metallic layer on the surface A of the polyester film. The film has    a) a coated surface A which is free of articles and            i) is at least 4 μm thick or        ii) is at least 50% of the thickness of the overall film ply; and            b) a particle-containing second layer having a comparatively rough surface which contains            i) at least 1% of individual particles of a certain polymer A and        ii) at least 1% of individual particles of a certain polymer B.        
There is again no information whatsoever in the document as to how the topography of such a film for the improvement of an oxygen barrier can be attained.
EP-B-0 088 635 describes a coextruded, biaxially oriented polyester film having at least two layers of which one layer A consists of thermoplastic resin and one layer B which comprises thermoplastic resin and fine particles. The film has a surface roughness Ra of the outer surface of layer A of less than 5 nm and the outer surface of layer B has either                a surface having a surface roughness Ra of from 5 to 40 nm and a plurality of depressions and a plurality of protrusions which are arranged in a certain arrangement, or        a surface having protrusions formed on a flat plane and whose surface is covered with a layer C composed of a lubricant and having a surface roughness Ra of from 5 to 40 nm.        
EP-A-0 514 129 describes a transparent multilayer film which comprises a primary layer substrate of polymer material which, on at least one of its surfaces, has a secondary layer of polymer material which has glass beads and silicon dioxide particles in certain concentrations and in certain size distributions. The secondary layer may be disposed on one or on both sides of the primary layer substrate. The film improves the opacity and the processing properties, but the document does not provide any teaching on the improvement of the gloss and of the barrier properties of the film. There is also no information whatsoever in the document as to how the topography of such a film for the simultaneous improvements of gloss and oxygen barrier is to be attained.
EP-A-0 604 057 describes a transparent multilayer film which comprises a primary layer substrate of polymer material which is substantially free of fillers and, on at least one of its surfaces, has a secondary layer of polymer material which contains silicone resin as a filler in a concentration of from 100 to 1000 ppm and an average particle diameter of from 1.5 to 12.5 μm. A disadvantage of the silicone particles is that they are comparatively expensive and do not constitute an acceptable solution for the packaging market. In addition, films which are equipped with such pigments have a more marked tendency to telescope on winding. There is likewise no information whatsoever in this document as to how the topography of such a film for the simultaneous improvement of gloss and oxygen barrier is to be attained.
As a consequence of the lacking smoothness of the surface A to be coated, the films described in the prior art are unsuitable for barrier coating by the CVD or by the PECVD method, or are at least in need of improvement. Likewise in need of improvement is the processing performance of these films on these high-vacuum units.